1. Field of the Invention
The invention relates to fluid flow systems and methods in general. More particularly, this invention relates to an apparatus and method for controlling and measuring the quality of a fluid mixture, such as steam, exiting branch conduits in a fluid distribution system.
2. Description of the Related Art
In the field of fluid flow measurement and control, it is often desirable to have uniform characteristics of that fluid flowing through multiple branches of a distribution system. For example, steam is often used to enhance the oil recovery from a reservoir. In the field of oil field technology, enhanced oil recovery operations are now common. Enhanced oil recovery operations recover oil or petroleum from marginal fields by stimulating flow of production of crude petroleum to an economically satisfactory level. Thermal methods of enhanced recovery introduce heat into a reservoir to improve the recovery of hydrocarbons.
One type of thermal method of enhanced oil recovery is steam flooding. In this method, steam is injected into the reservoir through an elaborate series of conduits or pipes. This steam supplies both heat and a fluid to the reservoir, which assists in extracting any remaining hydrocarbons from the reservoir. Steam generators supply saturated steam consisting of both gaseous steam vapor and liquid water.
A problem has been identified with providing steam to these reservoirs: the quality of steam varies. Steam quality at any given point depends on the steam quality generated at the source and the number and orientation of T-junctions. As steam is directed to the various wells in the field through the branching conduits, or T-junctions, the liquid and vapor phases will not split in equal proportion. This phenomenon is commonly referred to as phase splitting.
For instance, in a normal branch T-junction, the quality of steam diverted through the branch can vary depending on the inlet steam quality and the rate of flow through the main pipe. Also, normal branch T-junctions have a tendency to preferentially divert vapor from the main pipe into the branch. The steam quality is thus higher in the branch, and the steam quality in the main pipe diminishes downstream of the branch. Therefore, the quality of steam varies from branch to branch. But in most applications it is desirable to deliver steam of nearly equal quality to every branch in a distribution system because varying qualities of steam from branch to branch makes it difficult to efficiently manage a steam flood operation.
Several attempts to control fluid quality at multiple branches in a manifold systems are known. For instance, U.S. Pat. No. 5,250,104 (the "'104 patent") to Berger et al. describes a method and apparatus for controlling phase splitting at pipe junctions. As shown in FIG. 1 of the '104 patent, the '104 patent attempts to separate the stream of flow into its liquid and vapor phases upstream of the junction, and recombine them just downstream of the junction in proportion to the vapor mass rate flowing in each outlet leg. The underlying concept of the '104 patent is to size the piping elements which comprise the vapor branch, typically including an orifice plate, with respect to the piping components comprising the liquid bypass line, which can be as simple as a single length of tubing and can be constructed with or without means for measurement or adjustment, such that the vapor and liquid mixture ratio, steam quality, flowing to each outlet branch is constant.
However, the components for use in practicing the '104 patent must be designed for specific values of pressure and steam quality ranges. Thus, such a system must be sized according to the anticipated steam quality and pressures to be used in the field. Should these steam quality and pressures vary significantly, the '104 will not deliver steam of uniform quality as desired since '104 patent provides no passive adjustment mechanism. Further, for the '104 to be practiced successfully, it has been discovered that nearly 100% phase separation is necessary.
Another approach to control that steam quality was uniform in each branch of a steam distribution system is described in U.S. Pat. No. 5,415,195 to Stoy, et al. (the "'195" patent) entitled "Method and Apparatus for Controlling Phase Splitting at Branch T Pipe Junctions." The invention described in the '195 patent comprises a system that maintains steam quality in the branch despite changes in the inlet steam quality or changes in the rate of flow through the main pipe or the branch.
The steam quality delivered by the branches of systems utilizing the '195 patent will be determined by the geometry of the components used; however, for any given geometry, a balance between the vapor pressure drop and the liquid pressure drop will be established. This pressure drop balance is used to maintain branch steam quality.
Nearly perfect phase separation is difficult to achieve under a variety of flow conditions. The '195 patent attempts to solve this problem by having the branches take whatever vapor and the liquid they desire. Then, whatever is left remains in the main pipe, or "run." Thus, while the branches may deliver steam of uniform quality, the quality of steam remaining in the "run" is unknown. Since the run steam is not single phase, it cannot be measured with single-phase measurement devices. Also, pressure decreases will increase the vapor specific volume, resulting in decreased branch steam qualities. While active adjustment of control valves on branches of such a steam distribution system could possibly provide a desired steam quality, such active control is time-consuming, generally labor-intensive, and therefore impractical for most steam distribution systems.
Thus, it is desired to control a uniform quality of steam passing through every branch of a steam distribution, even if the incoming steam quality varies, without active adjustment. Also, it is desired to measure steam quality in every branch including the last branch of a steam distribution system, i.e. even the steam quality in the "run" is known.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.